The invention relates to an improved process for producing crystalline atorvastatin calcium which is known by the chemical name [R-(R*,R*)]-2-(4-fluorophenyl)-xcex2,xcex4-dihydroxy-5-(1-methylethyl)-3-phenyl-4-[(phenylamino)carbonyl]-1H-pyrrole-1-heptanoic acid hemi calcium salt.
Atorvastatin is useful as a selective and competitive inhibitor of the enzyme 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase, the rate-limiting enzyme that converts 3-hydroxy-3-methylglutaryl-coenzyme A to mevalonate, a precursor of sterols such as cholesterol. The conversion of HMG-CoA to mevalonate is an early and rate-limiting step in cholesterol biosynthesis.
Atorvastatin as well as some of its metabolites are pharmacologically active in humans and are thus useful as a hypolipidemic and hypocholesterolemic agent. The liver is the primary site of action and the principal site of cholesterol synthesis. Clinical and pathological studies show that elevated plasma levels of total cholesterol and associated triglycerides promote human atherosclerosis and are risk factors for developing cardiovascular disease.
U.S. Pat. No. 4,681,893, which is herein incorporated by reference, discloses certain trans-6-[2-(3- or 4-carboxamido-substituted-pyrrol-1-yl)alkyl]-4-hydroxy-pyran-2-ones including trans (xc2x1)-5-(4-fluorophenyl)-2-(1-methylethyl)-N,4-diphenyl-1-[(2-tetrahydro-4-hydroxy-6-oxo-2H-pyran-2-yl)ethyl]-1H-pyrrole-3-carboxamide.
U.S. Pat. No. 5,273,995, which is herein incorporated by reference, discloses the enantiomer having the R form of the ring-opened acid of trans-5-(4-fluorophenyl)-2-(1-methylethyl)-N,4-diphenyl-1-[(2-tetrahydro-4-hydroxy-6-oxo-2H-pyran-2-yl)ethyl]-1H-pyrrole-3-carboxamide, i.e., [R-(R*,R*)]-2-(4-fluorophenyl)-xcex2, xcex4-dihydroxy-5-(1-methylethyl)-3-enyl-4-[(phenylamino)carbonyl]-1H-pyrrole-1-heptanoic acid.
The above described atorvastatin compounds have been prepared by a superior convergent route disclosed in the following U.S. Pat. Nos. 5,003,080; 5,097,045; 5,103,024; 5,124,482; and 5,149,837, which are herein incorporated by reference and Baumann K. L., Butler D. E., Deering C. F., et al, Tetrahedron Letters 1992;33:2283-2284.
One of the critical intermediates disclosed in U.S. Pat. No. 5,097,045 has also been produced using novel chemistry, as disclosed in U.S. Pat. No. 5,155,251, which is herein incorporated by reference and Brower P. L., Butler D. E., Deering C. F., et al, Tetrahedron Letters 1992;33:2279-2282.
U.S. Pat. Nos. 5,216,174; 5,245,047; 5,248,793; 5,280,126; 5,397,792; 5,342,952; 5,298,627; 5,446,054; 5,470,981; 5,489,690; 5,489,691; 5,109,488; 5,969,156; 6,087,511; 5,998,663 and WO99/32434 which are herein incorporated by reference, disclose various processes and key intermediates for preparing atorvastatin.
It has been found that when the process for preparing atorvastatin calcium was scaled up to a commercial factory scale, drying was slow and difficult to optimize.
It was also found that wet crystalline atorvastatin calcium was susceptible to possible break up with physical attrition and furthermore had a propensity to form rock hard clods on mixing.
The object of the present invention is therefore to provide a process for producing crystalline atorvastatin calcium on a factory scale which routinely and consistently produces high quality material with reduced cycle time.
According to the invention there is provided a process for producing crystalline atorvastatin trihydrate hemi calcium salt comprising the steps of:
(a) reacting a mixture of atorvastatin lactone, methanol, and methyl tert-butyl ether with sodium hydroxide to form the ring-opened sodium salt;
(b) forming a product rich aqueous layer and an organic layer comprising methyl tert-butyl ether containing impurities;
(c) removing the organic layer comprising methyl tert-butyl ether containing impurities;
(d) extracting the product rich aqueous layer with methyl tert-butyl ether;
(e) adding an extra charge of methyl tert-butyl ether to a vessel containing the product rich aqueous layer in an amount of at least 1% w/v of the contents of the vessel;
(f) sealing the reaction vessel;
(g) heating the contents of the sealed reaction vessel to 47xc2x0 C. to 57xc2x0 C. in the presence of the extra charge of methyl tert-butyl ether which saturates the the crystallization matrix on heating;
(h) adding calcium acetate hemihydrate to the sealed reaction vessel to form atorvastatin trihydrate hemi calcium salt; and
(i) drying the isolated product in a vacuum pan dryer having an agitator which is continuously rotated at a speed of from 0.5 to 2 rpm.
It has been surprisingly found that this continuous agitation at a very low speed provides optimum drying conditions with very significant increased throughput while avoiding clod formation and particle attrition. As more solvents are evaporated from the cake in the dryer, the crystals are increasingly susceptible to attrition. The process ensures that no break-up occurs with physical attrition. Clod formation on mixing is also avoided.
In a particularly preferred embodiment of the invention, the agitator is substantially continuously rotated at a speed of approximately 1 rpm. This provides reduced drying time while ensuring uniform drying of the crystals, avoidance of clod formation and particle attrition. We have found that this uniform drying ensures that all water is evenly removed from the cake in the dryer.
In a preferred embodiment the vacuum in the pan dryer is maintained at from xe2x88x920.80 to xe2x88x920.99 bar.
Preferably, the isolated product is dried over a period of from 1 to 4 days, ideally over a period of from 1 to 2 days.
Atorvastatin lactone is saponified in a water/methyl alcohol/methyl tert-butyl ether (2-methoxy-2-methyl-propane; tert-butyl methyl ether) mixture with sodium hydroxide. The aqueous layer containing the sodium salt of atorvastatin is washed with methyl tert-butyl ether to remove small quantities of process impurities. A small aliquot of methyl tert-butyl ether is added to the crystallization matrix. Sodium-to-calcium salt metathesis with concurrent crystallization is accomplished by the slow addition of an aqueous calcium acetate solution to the sodium salt solution. To ensure crystallization simultaneous with addition, the reaction mixture is seeded with crystalline atorvastatin shortly after the start of the calcium acetate addition. The product is isolated by filtration and, after washing with water/methyl alcohol and water, is centrifuged and vacuum dried before milling to give crystalline atorvastatin as the trihydrate. The reaction scheme is shown below. 
We have surprisingly found that the drying of atorvastatin calcium has a sensitive window of moisture content of approximately 6% w/v where the drying has to proceed slowly in order to prevent the particles from breaking down. As more solvents are evaporated from the cake in the dryer, especially around 6% water, the particles are increasingly susceptible to attrition.
We have found that continuous agitation at approximately 1 rpm significantly reduces the drying time compared to an intermittent agitation technique, thereby increasing drying capacity while ensuring that the final dried product is within a consistent range for particle size and bulk density. This is in complete contrast to continuous medium speed agitation which results in clod formation and the risk of physical attrition and intermittent agitation which substantially increases the required drying time. Continuous agitation at 0.5 to 2 rpm is outside the normal design operating range of agitated pan dryers.